Electrical transmission lines and power generation equipment must be protected against faults and consequent short circuits, which can cause a collapse of the power system, equipment damage, and personal injury. It is the function of the protective relays, which monitor AC voltages and currents, to locate line faults and initiate isolation by the tripping of circuit breakers. Protective relays generally perform one or more of the following functions: (a) monitoring the system to ascertain whether it is in a normal or abnormal state; (b) metering, which involves measuring certain electrical quantities for operational control; (c) protection, which typically involves tripping a circuit breaker in response to the detection of a short-circuit condition; and (d) alarming, which provides a warning of some impending problem. Fault location, e.g., is associated with the protection function and involves measuring critical system parameters and, when a fault occurs, quickly making a rough estimate of the fault location and of certain characteristics of the fault so that the power source can be isolated from the faulted line. Thereafter, the system makes a comprehensive evaluation of the nature of the fault.
Modern protective relays employ microprocessors and/or digital signal processors (DSPS) to process the voltage and current waveforms measured on the protected transmission line (the term "transmission line" as employed herein is intended to cover any type of electrical conductor, including high power conductors, feeders, and transformer windings). Such processing may include the computation of a DFT. For example, U.S. Pat. No. 5,592,393, Jan. 7, 1997, titled "Method and System for Providing Protective Relay Functions," describes a system that uses the DFT function to compute instantaneous values of fundamental, second harmonic and fifth harmonic components. U.S. Pat. No. 5,172,329, Dec. 15, 1992, "Microprocessor Digital Protective Relay for Power Transformers," describes a system that uses the DFT function to compute voltage and current phasors.
The conventional DFT exhibits poor performance if the input signal contains a decaying DC component having a continuous frequency spectrum. Therefore, the DC signal component, or offset, is typically filtered out of the input signal before the DFT function is carried out. There are a number of the methods to deal with such DC offset, including the use of (1) a digital mimic circuit, (2) half-cycle and full-cycle compensation, (3) a parallel filter, and (4) a cosine filter. However, certain problems are associated with each of these methods. U.S. patent application Ser. No. 08/811,646, filed Mar. 5, 1997, "Protective Relay With Improved DFT Function," discloses an improved DFT function in which the decaying DC components are subtracted from the normal current and voltage phasors to yield modified phasors that are free of the effects of the decaying DC components. The present invention is directed to an alternative approach involving the use of an improved cosine filter.
Since distance relaying involves the use of voltage and current phasors to determine whether a fault is in the protected zone, it is imperative that the phasor estimates be as accurate as possible. The voltage signal may contain high-frequency components that can be filtered using an anti-aliasing filter and through least squares methods such as the DFT. However, the current may contain a decaying DC component what will cause the DFT to erroneously calculate the current phasor. The current phasor estimate will typically be inflated; therefore, the ratio of the voltage to the current will yield a small impedance value, causing the relay to trip when the fault is actually beyond the relay setting. This phenomenon is known as relay over-reach since the relay reaches beyond the setting and trips. Those skilled in the art recognize that the cosine filter provides one mechanism for removing the DC component of the current and thereby improving the current phasor estimate. Unlike the DFT, the cosine filter estimates only the cosine component of the current signal. The complete DFT is obtained by using two estimates shifted by 90 degrees, yielding the cosine and sine components. The cosine filter has been demonstrated for four samples per 60 Hz cycle. The principle is proven by approximating the decaying DC current term by the first two terms in the exponential series (a ramp function) as described in greater detail below. The object of the cosine filter is to sum the DC component such that it sums to zero. This concept can be extended to higher sampling rates. The problem is to select the cosine filter offset angle such that the DC ramp input sums to zero. The present invention is directed to providing a novel, improved cosine filter.